The scale on a map is 9cm : 1km. If the distance between two cities is 3km, how far apart in cm are the two cities on the map?
Answer: The scale means that every 9cm on the map corresponds to an actual distance of 1km. An actual distance of 3km is the same as 3 $\cdot$ 1km. The distance between the two cities on the map is 3 $\cdot$ 9cm, or 27cm.